1. Field of the Invention
The present invention relates to a light-emitting device using an organic compound and a display apparatus using the light-emitting device. More specifically, the present invention relates to an organic light-emitting device that emits light when an electric field is applied to a thin film including an organic compound, and a display apparatus using the organic light-emitting device.
2. Description of the Related Art
An organic light-emitting device (organic electroluminescence (EL) device; hereinafter, sometimes simply referred to as “device”) has been extensively researched and developed. Such an organic light-emitting device includes a pair of electrodes consisting of an anode and a cathode, and a plurality of organic compound layers including a light-emitting layer (hereinafter, simply referred to as “emission layer”) formed between the pair of electrodes. The organic compound layers each have a film thickness of about several tens of nanometers, and one of the electrodes reflects light. The other electrode functions as a light extraction electrode, and reflects a part of emitted light at an interface thereof with an external environment. Thus, an optical interference effect is developed remarkably in which light with a particular wavelength is resonated to be enhanced in an organic light-emitting device.
FIG. 1 is a schematic cross-sectional view of an ordinary organic light-emitting device. In FIG. 1, the organic light-emitting device includes a substrate 1, and a reflective layer 2, a transparent conductive layer (anode) 3, a hole-transporting layer 4, an emission layer 5, an electron-transporting layer 6, an electron injection layer 7, a semi-transmissive layer 8, a transparent electrode (cathode) 9 provided in the mentioned order on the substrate 1. Holes injected from the anode and electrons injected from the cathode are recombined in the emission layer by allowing a current to flow through the EL device, whereby light is emitted.
In such an organic light-emitting device, a resonator structure is constituted between the reflective layer 2 on the substrate and the semi-transmissive layer 8. Here, an optical path between the reflective layer 2 and the reflective interface of the semi-transmissive layer 8 is represented by L, a resonant wavelength is represented by λ, and an angle at which light emission from the device is recognized visually is represented by θ (it is assumed that the angle at which the light emission is recognized visually in front of the device, namely when recognized visually in a direction perpendicular to a light extraction surface of the device is defined as 0°). Further, in the case where the sum of phase shifts when emitted light is reflected by the upper and the lower electrodes is represented by φ (rad), and the order of optical interference is represented by m, when there is a relationship satisfying Equation 1 between the respective parameters, emission enhancement by resonance can be utilized.
That is, by bringing the peak wavelength of the emission spectrum (PL spectrum) generated by photoexcitation of the emission layer into conformity with the resonant wavelength, the intensity of the peak wavelength of the emission spectrum can be enhanced. Hereinafter, for differentiation from the PL spectrum, the emission spectrum taken out from the device is refereed to as “EL spectrum”.λ=2L cos θ/(m−φt/2π)  (Equation 1)wherein m represents a positive integer.
Incidentally, when emitted light is actually reflected by the respective electrodes, the sum of phase shifts φt varies depending upon the combination of an electrode material and an organic material constituting the reflective interface.
It can be seen from Equation 1 that the enhancement peak wavelength (resonant wavelength) λ varies depending upon the optical path L. Furthermore, when the angle (viewing angle) θ at which light emission is recognized visually with respect to the device increases, the cos θ value decreases, and the resonant wavelength λ is shifted to a shorter wavelength side. Therefore, in the case where the viewing angle is increased when the device having the optical path L is viewed, the resonant wavelength is displaced from the peak wavelength of the emission spectrum of the emission layer (PL spectrum), and the peak wavelength of the emission spectrum is not enhanced and further, is weakened. As a result, as the viewing angle increases, the emitted light looks darker.
Further, in the case where the viewing angle is increased, the resonant wavelength λ is shifted (blue shift) to a shorter wavelength side along with the decrease in the cos θ value, whereby the color of light emitted from the light-emitting device looks different, which is a problem.
The above-mentioned problem also applies to the case where the light-emitting devices are used as pixels of a display apparatus. Specifically, the above-mentioned problem also applies to the case of a display apparatus having a plurality of pixels with different emission colors (e.g., red, green, blue) as a pixel unit. In the case of such a display apparatus, there is another problem that the balance of a mixed color may be varied due to the change in color of each pixel when viewed obliquely. That is, when a light-emitting device that exhibits a red color in front thereof (namely, in a direction perpendicular to a light extraction surface of the device) is viewed obliquely, the light-emitting device exhibits an orange color. When a light-emitting device that exhibits a green color in front thereof is viewed obliquely, the light-emitting device exhibits a blue color. When a light-emitting device that exhibits a blue color in front thereof is viewed obliquely, the light-emitting device exhibits a darker blue color. Therefore, the white color (white balance) expressed by the color mixing of red, green, and blue is impaired.
Japanese Patent Application Laid-Open No. 2006-32327 discloses a light-emitting device in which portions having different optical paths are provided in one light-emitting device, and light emissions having different viewing angle characteristics are combined, whereby the change in characteristics depending upon the viewing angle is averaged and alleviated. This patent document discloses a device in which the optical path in the light-emitting device is changed, for example, a green-light-emitting device in which the resonant wavelength and the order at a viewing angle of 0° are each set to be 510 nm/m=2 and 570 nm/m=1.
Furthermore, Japanese Patent Application Laid-Open No. 2007-234581 discloses a light-emitting device and a display apparatus in which a plurality of portions with different optical paths are provided in one light-emitting device and between adjacent pixels having the same emission color, whereby the viewing angle characteristics are improved. The optical paths in this case are set to be changed within a range of 2% to 5% with respect to an average value of the different optical paths, and the order of optical interference is set to be the same value in the portions with any optical paths.
However, in the light-emitting device disclosed in Japanese Patent Application Laid-Open No. 2006-32327, since light emission spectra having two different peak wavelengths are combined, there is a problem that the full width at half maximum of the composite spectrum is enlarged, which makes it difficult to improve the color purity of emission color.
Furthermore, in the light-emitting device disclosed in Japanese Patent Application Laid-Open No. 2007-234581, since the change in optical path in one light-emitting device or between adjacent pixels is about 2 to 5% of an average optical path, there is a problem that the effect of improving the viewing angle characteristics is small.
The present invention has been accomplished in view of the above-mentioned problems, and it is, therefore, an object of the present invention to provide an organic light-emitting device that has high color purity of light emission and enables excellent color reproduction, while having wide viewing angle characteristics. Furthermore, it is another object of the present invention to provide a display apparatus having the organic light-emitting device.